Photo Blog vs Flickr/500px. What's the Difference?

After browsing through The Lens, I've found that there are a lot of people more interested in maintaining a photo blog on Tumblr than an account at Flickr or 500px. But, as someone who's looking to get into photography more, what's the difference? I know that photo-sharing services like 500px are meant for more exposure and "best work", but what I'm more interested about is, what's the benefit of a photo blog in which you'd post whatever you'd post in something like Flickr or 500px anyway?